1. Field of the Invention
The present invention relates to a speed changing control system for a vehicle including a speed changing device for speed-changing a rotational power from an engine and outputting the resultant speed-changed power, a speed changing control unit for setting a speed change ratio of the speed changing device and an engine control unit for setting an engine rotational speed of the engine.
2. Description of the Related Art
As described above, with a speed changing control system of the above-noted type in operation, through coordinated controlling of the engine control unit and the speed changing control unit, it becomes possible to maintain the vehicle speed at a constant value in both low and high engine rotational speed ranges.
For instance, JP 5-338474 A (paragraph [0004], FIG. 10), shows a movable agricultural machine in which a speed changing actuator for actuating a stepless speed-changing mechanism is provided so that the traveling speed (vehicle speed) is changed steplessly by the control of this actuator. More particularly, in this movable agricultural machine, a rotation sensor and an accelerator actuator are provided for effecting respectively detection and adjustment of the engine rotation. In operation, by operating the respective actuators in such a manner as to obtain a desired traveling speed, the speed change ratio of the stepless speed changing mechanism and the engine rotation are controlled in correlation (coordination) with each other. And, at the time of light-load traveling, the engine is set to a low rotational speed for a lower fuel consumption rate to achieve economical traveling whereas at the time of high-load traveling, the engine is set to a high output to allow traveling at a fixed desired speed.
If the rotational speed of the engine can be lowered with keeping the vehicle speed constant, the fuel consumption rate can be lowered correspondingly, thus being advantageous for energy saving. However, as spare or excess amount (un-used portion) of the engine torque is diminished, the risk of occurrence of engine stall becomes higher, thus inviting inconvenience of vehicle travel becoming unstable. The spare amount of engine torque varies depending on e.g. traveling condition, the work condition. In the case of steep slope traveling or muddy road traveling, the spare amount of engine torque is limited as a matter of course. The driver can feel (grasp) such situation or condition, but the conventional speed changing control system is not equipped with any operational line or mechanism capable of operably associating such driver grasped condition with the energy saving driving described above.